tcp: Don't clear ACK_TO_TAP_DUE if we're advertising a zero-sized window

We correctly avoid doing that at the beginning of tcp_prepare_flags(),
but we might clear the flag later on if we actually end up sending a
"flag" segment.

Make sure we don't, otherwise we might delay window updates after a
zero-window condition significantly, and significantly affect
throughput.

In some cases, we're forcing peers to send zero-window probes or
keep-alive segments.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 2 insertions, 1 deletions

diff --git a/tcp.c b/tcp.c
index 4a886ae..d91db22 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1282,7 +1282,8 @@ int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
 	th->fin = !!(flags & FIN);
 
 	if (th->ack) {
-		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap))
+		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap) &&
+		    conn->wnd_to_tap)
 			conn_flag(c, conn, ~ACK_TO_TAP_DUE);
 		else
 			conn_flag(c, conn, ACK_TO_TAP_DUE);